This invention is generally directed to a portable blood pressure measuring device providing accurate, essentially continuous readings without requiring affirmative action of the user, and, more particularly, to a device using a transducer and self-contained circuitry to calculate values for systolic and diastolic blood pressure. The continuous measurement of a patient's blood pressure is a virtual necessity to fully comprehend the dynamics of hypertension and other medical problems. Although the medical community does not know all the factors involved in raising and lowering of blood pressure, it is generally agreed that emotional stress, physical exertion and the chemical composition of an individual's blood, such as sodium concentration, affect one's blood pressure.
For those individuals suffering from chronic high blood pressure, there are many medications that can be taken. However, specifying the most appropriate dosages and their optimum timing is currently problematic. In chronic cases of high blood pressure, when a sufferer feels an attack, there are medications that instantaneously reduce the blood pressure of the individual.
However, most individuals are not aware when a gradual, or even sudden, rise in blood pressure has occurred, though such rise in blood pressure can cause heart attacks, aneurysms, strokes and other serious damage. Accordingly, there is a need for a monitoring device that can quickly and continuously monitor a person's blood pressure without inconveniencing or inhibiting the ability to conduct daily life, and without requiring affirmative action of the user during normal use.
Previous monitor systems have relied upon inflation techniques mimicking the original sphygmomanometer based on Riva Rocca's principle in which auscultatory measurements of blood flow are made to determine the systolic and diastolic blood pressure. When the inflation process is automated and mechanized, better control and consistency can be gained, but new possibilities for further error are also introduced. By eliminating the repeated use of the inflation approach, and the inherent problems associated therewith, and using a calibration technique, increased accuracy can be provided.
A non-invasive approach known as the tracking cuff principle relies upon a hydraulic servo control system to maintain a finger arterial volume constant, in which case the counter cuff pressure follows the intra-arterial blood pressure giving an instantaneous blood pressure. This, however, has not proved accurate and because it requires long periods of cuff pressure, it can be painful and uncomfortable to the patient.
Another approach which also has not proved to be accurate and does not provide continuous values of blood pressure is the annular inflatable cuff for placement about a patient's finger. The cuff is inflated to the appropriate pressure and light emitting diodes are provided on the inside of the cuff and a photoelectric transducer is provided on the opposite inner wall of the cuff. The light is partially transmitted through the patient's finger to the photoelectric transducer which measures arterial volumetric changes in the finger. These are transmitted to a microprocessor which controls the inflation and deflation of the cuff around the finger and calculates systolic and diastolic pressure using the Hardy and Collins compliance model in which the pressure-volume relationship of blood vessels is described by an algorithm. This technique has been found intermittent and not dependable when tested (U.S. Pat. No. 4,846,189).
Non-invasive blood pressure monitoring transducers are known in the prior art as exemplified by U.S. Pat. No. 4,423,738 (Newgard). The Newgard patent is directed to a transducer array for arterial tonometry. This invention includes a transducer provided in a case similar to that of a conventional wristwatch. The case is held in place over the radial artery in a human wrist by an expansion band. The transducer is electrically wired to a separate housing that includes the electronic circuitry that converts the electric information to a digital readout. The cord connecting the wrist piece to the housing also includes a source of gas or air to create a constant pressure in the case. The transducer case is required to exert enough pressure against the artery wall to flatten the artery wall. Accordingly, unlike the present invention, this invention requires an external source of gas or air to flatten the user's artery.
A second blood pressure monitoring method and apparatus is disclosed in U.S. Pat. No. 4,802,488 (Eckerle). The Eckerle patent describes a non-invasive method of measuring intra-arterial blood pressure using an array of transducer elements. A set of blood pressure readings is stored in a computer along with pulse amplitude values and information concerning the subject (i.e., age, weight, arm and wrist diameter). From the underlying information, the diameter of the artery can be estimated. Once the diameter of the artery is estimated, the computer knows what portion of the transducer is required to measure arterial pressure and pulse amplitude. The required "hold-down pressure" is supplied by an air pressure source. Accordingly, this invention also suffers from the inability to sample arterial blood pressure without the use of air pressure in a bladder exerting pressure.
Another blood pressure monitoring system is disclosed in European Patent Application No. 0.297,146 which discloses a blood pressure monitoring system which incorporates a conventional cuff and a pulse monitor used in combination to produce a non-continuous blood pressure monitoring system. This system is insufficient in that it requires frequent inflation of the cuff in order to provide a blood pressure reading.
An accurate, non-invasive, continuous method of measuring blood pressure is desirable to allow individuals to monitor their own blood pressure providing for more accurate use of anti-hypertensive medication and preventing overmedication and its side effects. Even in normotensive individuals, there are those with labile hypertension which has gone undetected. Diseases associated with episodes of elevated blood pressure are now being detected with the use of Computerized Axial Tomography ["CAT"] and Magnetic Resonance Imaging ["MRI"] such as multi-infarction dementia in individuals not known to be hypertensive suggesting that monthly visits to a physician's office may not be frequent enough to ensure that an individual is not periodically hypertensive. It is becoming evident that the need for more frequent blood pressure monitoring in the so-called normotensive population is greater than heretofore recognized. Further, having a record of a patient's blood pressure at various points during the day may help in diagnosis and treatment of patients suffering from hypotension as well as from hypertension, and from disorders related to non-normal blood pressure behavior. Hypotension can result in fainting, syncope, or general feelings of weakness or lethargy, and can lead to potentially dangerous losses of functioning. Where patterns of increasing or decreasing blood pressure can be established and treated, they can be controlled before they can cause damage to the individual.